Correlation between F_Hα and F_Y for the redshift bin centred at $\overline{z}=1$ of the Flagship spectroscopic mock catalogue. A continuous line denotes the flux limit of the Euclid Wide Survey, while a dashed line highlights a Y_E-band flux cut, splitting the sample in two equally-populated bright and faint populations. The reference fluxes are F_Hα0 = 1 erg cm⁻² s⁻¹ and F_Y0 ≡ 1 erg cm⁻² s⁻¹ Hz⁻¹. The colour bar highlights the density of objects (in arbitrary units).